The present invention relates generally to photographic film processing systems. In particular, the present invention is a densitometer for providing information representative of the density (degree of lightness or darkness) of images on the film.
Electronic imaging systems are widely used in the medical field. Imaging systems of this type include a Computed Tomography (CT), Magnetic Resonance (MR) or other type of scanner to generate the image data, and an imager responsive to the data for exposing the image on photographic film. The film is subsequently developed in a film processor to produce a hard copy of the image.
The image on the developed film is formed from areas which vary in lightness and darkness. In order for medical personnel to make accurate diagnoses, the areas of lightness and darkness must accurately represent the image data generated by the scanner. To this end, densitometers are typically used for determining the density, or degree of lightness or darkness, of various portions of the film. Information representative of measured film density is then used by the imager and/or film processor to optimize the density of subsequently imaged and developed film.
The density (D) of an exposed and developed area of film is defined as the common logarithm of the inverse of the transmittance (T) (i.e., D=log.sub.10 (T.sup.-1). The transmittance is defined as the portion of incident light impinging upon one side of the film which passes through the film. Known densitometers typically include a light emitting subsystem with a light source for impinging light on a selected area of the film. A light detecting subsystem including a photosensitive element such as a photodiode is positioned on the other side of the film, and detects the portion of light passing through the film. Circuitry coupled to the light detecting subsystem determines the transmittance, and generates signals representative of the film density in accordance with the equation given above. The densitometer circuitry typically includes a logarithmic amplifier since a large range of signals must be detected and amplified. By way of example, the light intensity reaching the photodetector is down by a factor of 2000 for a density of 3.3 from the light intensity corresponding to a density of 0.0. Unfortunately, logarithmic amplifiers are relatively costly and are not available in monolithic form.
Other densitometers are discussed and disclosed in the Thomas et al. U.S. Pat. No. 4,424,589. In the Field And Background Of The Invention section it is noted that prior systems used either an analog method of logging, or a digital method using an analog-to-digital converter and a memory lookup table containing the logarithmic values. The scanner system disclosed in the patent includes three sets of analog-to-digtal converters associated and memory devices to convert a range of analog signals into corresponding digital density values. The range of analog signals is divided into three amplitude bands. Each set of converters and memory devices operates on signals within one of the bands to reduce the wasted resolution capacities of the converters and memories at high intensity levels.
The Bellanger et al. U.S. Pat. No. 4,700,058 discloses an imaging system which incorporates feedback control techniques to provide control of film density. A first feedback loop monitors light intensity from the film writing device and provides a feedback signal to a variable gain amplifier to maintain the light output of the writing device at a desired level. A second feedback control loop monitors the density of the exposed and developed film, and provides feedback signals to a variable gain amplifier. By changing the gain of the amplifier, the image data signal is adjusted in such a manner as to correct for any deviations from a desired film density. The system also includes a digital memory for storing a lookup table of film reference density settings as a function of voltage levels.
The Volent U.S. Pat. No. 4,757,334 discloses a film imaging and processing system with a density correction system. Film used with the system includes a density calibration strip. Densities of the calibration strip are monitored by a densitometer after the film has been developed. The film densities of the developed calibration strip are then compared to a predetermined standard to determine any variations. In response, a correction signal is applied to the scanner to adjust the density on subsequently exposed sheets of film.
It is evident that there is a continuing need for improved densitometers for photographic film imaging and processing systems. The densitometers must quickly and accurately generate signals representative of film density. To be commercially viable, the densitometers must also be capable of implementation with relatively inexpensive electronic components.